1. Field of the Invention
The present invention is generally directed to tools and methods employed to obtain downhole measurements in a subterranean well bore.
2. Description of the Related Art
Oil and gas wells are formed by a rotary drilling process. To that end, a drill bit is mounted on the end of a drill string which may be very long, e.g., several thousand feet. At the surface, a rotary drive mechanism turns the drill string and the attached drill bit at the bottom of the hole. In some cases, a downhole motor may provide the desired rotation to the drill bit. During drilling operations, a drilling fluid (so-called drilling mud) is pumped through the drill string and back up-hole by pumps located on the surface. The purpose of the drilling fluid is to, among other things, remove the earthen cuttings resulting from the drilling process.
Weight-on-bit (hereinafter WOB) is generally recognized as being an important parameter in controlling the drilling of a well. The weight is applied to the bit by a string of heavy drill collars that is attached immediately above the bit and suspended in the borehole on smaller diameter drill pipe. In conventional drilling practice, the entire length of the drill pipe and an upper portion of the drill collar string are suspended at the surface from the derrick in tension, so that the amount of WOB can be varied by changing the indicated surface hookload. Properly controlled WOB is necessary to optimize the rate that the bit penetrates a particular type of earth formation, as well as the rate of bit wear. WOB also is utilized in controlling the direction of the hole, and accurate measurement thereof can be used in analyzing drilling rate “breaks” indicative of entry of the bit into more porous earth formations. Thus, precise and accurate measurements of the WOB parameter may be important in the drilling process. Torque also is an important measure useful in estimating the degree of wear on the bit, particularly when considered together with measurements of WOB.
In the past, WOB measurements have sometimes been made at the surface by comparing indicated hookload weight to off-bottom weight of the drill string. However, surface measurement of WOB is not always reliable due to the drag of the drill string on the borehole wall, and other factors.
In other cases, a strain gauge bridge positioned in a downhole tool has been used to obtain various data, including WOB measurement data. During drilling operations, there is a pressure difference between the internal pressure within the drill pipe and the external pressure in the well bore annulus between the drill pipe and the well bore. This pressure differential may be quite large, e.g., on the order of approximately 200-800 psi for a typical well. A large percentage of pressure differential is due to the pressure drop as the drilling fluid circulates throughout the drill bit. The downhole pressures result in strains that act in the same sense as the axial strains associated with the WOB, thereby creating the possibility that the strains associated with the downhole pressures can be misinterpreted as reflecting WOB values. While the differential pressure may be on the order of 200-850 psi, the overall pressure may be as high as approximately 10,000 psi. Pressures of this magnitude may cause massive errors in WOB measurements.
At least theoretically, the strains induced by the downhole pressures can be compensated for by various pressure correction factors that are based upon various calculations. However, there are several drawbacks to such a methodology. For example, if the strains from the downhole pressures are combined with the strains from the WOB, the overall strain values that may be obtained are much higher than the strain value for the WOB alone. In turn, this requires that a data acquisition system used to obtain such strain data must have a relatively larger analog input range, thereby resulting in a lower resolution of the strain values of interest. Second, if the internal and external pressures are not measured, or at least not accurately measured, it is very difficult to accurately apply pressure correction factors. Moreover, such pressure correction factors have inherent inaccuracies that, all other things being equal, would preferably be avoided.
The present invention is directed to an apparatus and methods that may solve, or at least reduce, some or all of the aforementioned problems.